1. Field of the Invention
This invention relates to a method of manufacturing a hollow engine valve for an internal combustion engine.
2. Prior Art
Generally, heat-resistant steel is used as a material for an engine valve of this type, and there are many kinds of hard-cutting materials in such heat-resistant steel. Therefore it is difficult to cut a hollow hole into such material. In view of this, Japanese Patent publication No. 29509/76 has disclosed a method in which a metal insert is inserted into and fixed to that section of a pipe material where a mushroom portion is to be formed, and then the pipe material is subjected to hot upsetting and then to hot press-shaping. The hot upsetting requires a long time because cf the resistance heating of the material, and therefore has the problem that the productivity is inferior. Japanese Laid-Open (Kokai) Patent Application Nos. 179833/87 and 179835/87 disclose a method in which a hole is formed in a central portion of a valve material or stock by rear extrusion, a core of ceramics or a low melting-point material, such as copper or graphite is filled in this hole, and after a shaping operation, the core is removed by melting or crushing, thereby forming a hollow portion. With this method, however, before the mushroom-shaped portion is shaped by pressing, the separate step, that is, the rear extrusion for creating the hole, is required. Thus, this method needs more steps than the method using the pipe material, and time and labor are required for removing the elongated core by heat-melting or crushing after the shaping operation. Therefore, such a method suffers from a problem that it is not suited for mass-production.